Method of removing chlorine compounds from gases obtained by combustion of waste liquors



United States Patent 3,269,095 METHOD OF REMOVHNG CHLORINE COM- POUNDSFROM GASES UBTAINED BY COM- BUSTION 0F WASTE LIQUORS Rolf Briinnland,Ornskoldsvilr, Sweden, assignor to M0 Och Domsjo AB, Ornskoldsvik,Sweden, a corporation of Sweden No Drawing. Filed Nov. 26, 1963, Ser.No. 326,262 Claims priority, application Sweden, Dec. 5, 1962, 13,130/624 Claims. (Cl. 55-71) The present invention relates to a method ofreducing the accumulation of chlorides in chemical recovery processesbased on the combustion of waste liquors resulting from the pulping ofcellulosic materials by the sulfite or sulfate pulping methods.

A disadvantage of a closed chemical recovery system is that anaccumulation of undesirable substances easily occurs. With regard tocorrosion problems, chlorides are the most objectionable substances. Aslong as the recovery processes are directed only to the base used in thepulping, the accumulation of chlorides is low, because a considerableportion of the amount of chlorides supplied e.g. with fresh water orwood is exhausted with the combustion gas from the combustion of theconcentrated waste liquor.

Thus, e.g., in a sulfite pulp mill, operating on sodium base, it wasobserved that 40% to 60% of the amount of chloride supplied to thecombustion furnace left with the combustion gases as hydrogen chloride.

However, if it is desired to expand the chemical recovery system toinclude also the sulfur dioxide in the combustion gases by introducing acombustion gas scrubber, the concentration of chlorides in the systemwill tend to reach values which may be up to times higher than thevalues achieved without chemical recovery, due to the fact thatchlorides will be taken up from the combustion gases in the scrubber andthus returned to the system.

Therefore, it is desirable to remove the hydrogen chloride from thecombustion gas in a separate preliminary step, before the absorption ofsulfur dioxide starts. In conventional combustion gas scrubbers oneoperates as a rule at gas velocities of between 1 and 2 meters persecond and liquid loadings of above 0.5 liter per cubic meter of gas(normal conditions) to achieve a sufiicient degree of wetting in thegas.

Degree of wetting refers to the portion of the gas which contacts thewashing liquid. Thus, e.g., 100% degree of Wetting means that nosuperheated gas passes the first washing step of a combustion gasscrubber.

A conventional washing system would involve great difiiculties forwashing out chlorides, because with the great amounts of water involvedit would be necessary to recirculate the washing liquid in order to keepthe absorption of sulfur dioxide at a sufliciently low level. This wouldinvolve high requirements for corrosion resistance in pumps, pipes,nozzles etc.

Another problem has to do with the tendency of hydrogen chloride to formwith water fine droplets of hydrochloric acid, aerosols, which arediflicult to absorb in washing liquids of various types. Therefore, aconventional combustion gas scrubber will give a very low degree ofabsorption in the first chloride separating step, and a major portion ofthe chlorides will be absorbed in the following alkaline sulfur dioxideabsorption step.

Surprisingly, it has been found, however, that it is possible to removethe hydrogen chloride from the combustion gas before recovery of sulfurdioxide, using a small volume of washing liquid and thus a low level ofabsorption of sulfur dioxide, by supplying the water to a washingsection having a higher gas velocity than the conventional, preferably10 to 20 meters per second. The supply of water can then be reduced to0.05 to 0.2 liter per cubic meter of saturated gas (normal conditions)without the degree of wetting being lower than in a conventionalcombustion gas scrubber.

The temperature of the water may vary within wide limits, since thetemperature of the gas after washing is only slightly influenced by thetemperature of the injected water. In all cases, the gas achieves afterwashing a temperature close to its original dew point. Therefore, asuitable temperature of the water is from about 0 C. to about C. Inparticular, however, it has been found suitable to supply the water at arelatively low temperature, preferably from 0 C. to about 20 C. and asfine droplets. Thereby, the superheated gas which normally has atemperature of to 400 C. is cooled, and by the resulting condensation ofwater vapor, the size of the hydrogen chloride droplets is increased, sothat the separation possibilities are considerably improved. Whensupplying water at a low temperature, the washed gas obtains atemperature slightly below the dew point of the original gas mixture.The greater the amount of water and the lower its temperature, the lowerthe gas temperature will be. In practice it has been found possible tolower the temperature of the washed gas to 0.2 to 20 C. below the dewpoint of the original gas mixture.

The droplet size at which the water is injected, is also of importancefor the efficiency of the chloride absorption process. If the dropletsused are too large, the contact surface between the gas and the liquidwill be too low and the chloride absorption will be low. If the dropletsused are too small the degree of absorption will be higher, but theresulting mist will be difiicult to separate and will accompany thewashed gas to the following washing step, absorbed in the washing liquidused therein and cause corrosion. A suitably average droplet size is 0.1to 0.5 millimeter. A preferred droplet size is 0.15 to 0.30 millimeter.

A suitable droplet size is obtained by a suitable selection of injectionnozzles and by controlling the pressure of the water supplied thereto.

Separation of the hydrochloric acid containing washing liquid can takeplace in a spray condenser of conventional type.

The addition of water for the removal of chlorides can take place, e.g.,in a gas conduit leading to the combustion gas scrubber, wherein foreconomical reasons the gas velocity is often selected between 10 and 20meters per second. If other gas velocities in the gas conduit leading tothe scrubber are used, it is suitable to provide, by a change of itscross-section (constriction or expansion) a gas velocity of between 10and 20 meters per sec- 0nd in the combustion'gas conduit adjacent to thescrubber. Experiments have shown that. the best possible degree ofwetting of the gas is achieved by using so-called plan nozzles, whichdeliver a flat, horizontal spray, and which are disposed in slotsadjacent the gas conduit and which introduce the water in a plane atright angles to the direction of flow of the gas. Another suitablemethod is also to use so-called full cone nozzles which are placed iiithe gas flow so that the water is introduced substantially opposite tothe direction of flow of the gas. With these positions of the nozzles, asatisfactory degree of wetting will be obtained, and at the same timethe nozzles will be protected from contact with the superheatedcorrosive flue gas. If it is preferred to carry out the removal ofchlorides from the combustion gases in the same tower as used for thecontinued washing of the combustion gases it is possible to provide thistower with constriction means such that the desired gas velocity of 10to 20 meters per second is obtained, whereupon the washed gas is allowedto expand to the lower gas velocity suitable for the followingabsorption steps.

The invention is illustrated, but not limited by the following example,wherein gas percentages are by volume.

Example A combustion gas from combustion of sodium sulfite waste liquorhad the composition 63.4% of nitrogen, 21.4% of water, 11.0% of carbondioxide, 3.9% of oxygen, 0.3% of sulfur dioxide and 0.003% of hydrogenchloride. Its temperature was 125 C. prior to the treatment in thecombustion gas scrubber. If this gas were saturated with water of 5 C.temperature, the gas mixture would have a dew point of 644 C.

By so called plan nozzles disposed in slots adjacent the horizontal gasconduit, washing water was introduced substantially horizontally and atright angles to the direction of gas flow. By varying the nozzle sizeand operating pressure, the addition of water and the droplet size werevaried, while the gas velocity was maintained at meters per second.

The dew point of the gas mixture was lowered by an addition of 0.1 literof water of +5 C. temperature per cubic meter of gas (normal conditions)to 63.5" C., i.e. 0.9 C. below the dew point of the saturated gas.

In a conventionally carried out chloride washing at a gas velocity of1.5 meters per second and a liquid loading of 0.5 liter per cubic meterof gas (normal conditions), one obtains with the above gas mixture aloss of 38% S0 based on the amount of S0 supplied, due to absorption inthe washing water. The loss can be reduced by recycling the washingwater, but in that case serious corrosion problems will occur.

The term water-soluble chlorine compounds is used in the claims toencompass chlorides, that is chlorine containing salts, and hydrochloricacid.

I claim:

1. A method of removing water-soluble chlorine compounds from gasesobtained by combustion of waste liquors from the pulping of cellulosicmaterials by the sulfite or sulfate pulping methods, which comprisescontacting superheated gases resulting from said combustion at atemperature within therang from about 100 to about 400 C., and at avelocity Within the range from about 10 to about 20 meters per secondwith cold water at a temperature within the range from about 0 to about20 C., and at an average droplet size within the range from about 0.1 toabout 0.5 mm. in an amount sutficiently in excess of the amount requiredto saturate the gas to lower the temperature of the gas to about 0.2 toabout 2 C. below the dew point of the gas saturated with water vapor.

2. A method as in claim 1, in which the excess of water is 0.05 to 0.2liter per cubic meter of gas at at mospheric conditions.

3. A method as in claim 1, in which the cold water is introduced in theform of a flat spray in a direction substantially in a plane at rightangles to the direction of gas flow.

4. A method as in claim 1, in which the cold water is introduced in theform of a full cone spray, in a direction countercurrently to the gasflow.

References Cited by the Examiner UNITED STATES PATENTS 1,821,064 9/1931Skogrnark et al -73 2,207,774 7/1940 Barthelerny 55-89 X 2,221,066 11/1940 Kahle 55- 73 2,252,536 8/1941 Wiley 55-71 2,598,116 5/1952 Dubois55- -73 FOREIGN PATENTS 67,867 5/1944 Norway.

REUBEN FRIEDMAN, Primary Examiner.

J. ADEE, Assistant Examiner.

1. A METHOD OF REMOVING WATER-SOLUBLE CHLORINE COMPOUNDS FROM GASESOBTAINED BY COMBUSTION OF WASTE LIQUORS FROM THE PULPING OF CELLULOSICMATERIALS BY THE SULFITE OR SULFATE PULPING METHODS, WHICH COMPRISESCONTACTING SUPERHEATED GASES RESULTING FROM SAID COMBUSTION AT ATEMPERATURE WITHIN THE RANGE FROM ABOUT 100 TO ABOUT 400*C., AND AT AVELOCITY WITHIN THE RANGE FROM ABOUT 10 TO ABOUT 20 METERS PER SECONDWITH COLD WATER AT A TEMPERATURE WITHIN THE RANGE FROM ABOUT 0 TO ABOUT20* C., AND AT AN AVERAGE DROPLET SIZE WITHIN THE RANGE FROM ABOUT 0.1TO ABOUT 0.5 MM. IN AN AMOUNT SUFFICIENTLY IN EXCESS OF THE AMOUNTREQUIRED TO SATURATE THE GAS TO LOWER THE TEMPERATURE OF THE GAS TOABOUT 0.2 TO ABOUT 2*C. BELOW THE DEW POINT OF THE GAS SATURATED WITHWATER VAPOR.